Conventionally, continuously variable transmissions (CVTs) capable of continuously changing a gear ratio have been known. Various types of continuously variable transmissions such as belt-type, chain-type, and toroidal-type have been put to practical use. Continuously variable transmissions can shift smoothly, and have a gear ratio with a high degree of freedom. Thus, continuously variable transmissions are generally excellent in fuel consumption properties.
How to control a continuously variable transmission varies depending on the type of vehicle, the type of engine, and the like. Therefore, it is necessary to tune a continuously variable transmission for each type of vehicle and for each type of engine. Thus, vehicle development efficiency may be degraded, or cost of manufacturing a control system may be increased.
In view of such a problem, a vehicle control system that is appropriate for integrated control of a vehicle equipped with a continuously variable transmission, and is suitable for standardization, rationalization, and the like of a plurality of configuration units in the vehicle.
Japanese Patent Laying-Open No. 2002-5277 (Patent Document 1) discloses a vehicle control system in which a transmission gear ratio computation portion computes a command value of a transmission gear ratio of a continuously variable transmission mounted in a vehicle, a torque detection portion detects input torque to the continuously variable transmission, and a transmission gear ratio control portion controls the transmission gear ratio of the continuously variable transmission based on the command value and a detected value. In the vehicle control system, a transmission gear ratio control unit having a function as the transmission gear ratio control portion and one or a plurality of functional units having functions as the transmission gear ratio computation portion and the torque detection portion are configured as units different from each other, and connected via a communication line capable of transmitting data.
According to the vehicle control system described in Japanese Patent Laying-Open No. 2002-5277, computation of the transmission gear ratio is not performed in the transmission gear ratio control unit, and the transmission gear ratio control portion controls the transmission gear ratio of the continuously variable transmission simply based on the command value of the transmission gear ratio and the input torque transmitted to the transmission gear ratio control unit via the communication line. Thereby, even if the transmission gear ratio control unit is mounted in a vehicle of a different type or having a different type of engine, there is no need to tune the transmission gear ratio control unit accordingly. Therefore, the continuously variable transmission and the transmission gear ratio control unit can be configured independently of the vehicle type. Thus, the continuously variable transmission can be standardized (commonalized) in terms of both hardware and software. On the other hand, in the transmission gear ratio computation portion and the torque detection portion, it is required to change control variables and the like used by these portions depending on the vehicle type. If these portions are configured as one functional unit, matters to be changed in terms of both hardware and software can be centralized.    Patent Document 1: Japanese Patent Laying-Open No. 2002-5277